Thermal processing of whole soya beans: Studies on the inactivation of antinutritional factors and effects on ileal digestibility in piglets

Two experiments were conducted to investigate the effects of steam processing on the nutritional improvement of full-fat soya beans. In a kinetic study, soya beans were toasted at 102, 120 and 134°C for various durations (19 temperature/time combinations [Tt]). The raw and processed soya beans were measured for their functional lectins, trypsin inhibitor activity (TIA), urease activity (UA) and protein dispersibility index (PDI). The raw soya beans contained 7638 μg g−1 functional lectins, 23.44 mg g−1 TIA, 2.22 UA (ΔpH) and 85.7% PDI. These criteria all decreased as the heating time increased. Of all criteria measured, the level of functional lectins was found to be most effectively inactivated by high temperature. second experiment, 35 piglets of approximately 30 kg liveweight were used. The animals were fitted with a post-valvular T-caecum (PVTC) cannula and divided into seven groups, which were fed a control diet and six experimental diets. The diets consisted of 80% control diet and 20% processed soya beans which were treated under the following conditions: 102°C for 10 min, 102°C for 20 min, 102°C for 40 min, 120°C for 2 min, 120°C for 7.5 min and 134°C for 1.5 min. The apparent ileal digestibilities of the soya beans processed in these conditions [Tt] were: 37.0, 49.1, 61.3, 48.2, 57.0 and 56.8%, respectively, for dry matter (DM); 51.1, 68.3, 81.2, 70.3, 82.0 and 82.2%, respectively, for nitrogen (N); 82.2, 84.1, 89.2, 89.6, 93.0 and 91.8%, respectively, for ether extract. The apparent digestibilities of DM and N increased as the level of antinutritional factors decreased. It appears that a certain amount of energy is required to achieve the inactivation of antinutritional factors, either by treatment at a high temperature for a short time (e.g. 134°C for 1.5 min), or for a longer time at a lower temperature (e.g. 102°C for 40 min). It was noticed that the residual TIA content of the processed soya beans was correlated to the variation in digestibility among the animals, which indicates that there are considerable differences between individual animals in their responses to the soya bean antinutritional factors.